Electronic States
Work blog of Atomium Amplification in Philadelphia, PA
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There are four versions of this amp that I’m aware of:
1. Non-master volume, early (the “good one”)
2. V4B (for bass)
3. Master volume version, later
4. Distortion channel version
I’ve repaired examples of all of them except the distortion version, which I’ve never seen in person. I don’t know if there was a version with distortion AND master volume. The VT-22 is the combo version, which I rarely see. Numbers 1-3 above are 99% alike, despite what you’ve heard to the contrary. The distortion model is quite different.
This amp had severe heat damage at some point, which had been patched here and there in previous repairs. The insides were a hodgepodge of parts-on-hand repairs, drifted values, and intermittent parts. PCBs were all filthy and looked like they had a combination of cigarette tar and carbonization from either fire or exploding parts. Many components measured ok but were visibly toasted. One of the preamp cover plates was gone and so were many of the chassis screws. The power and standby switches were burned up and intermittent. The screen resistors crumbled when I touched them.
About 85% of the PCB-mounted parts ended up replaced, including every capacitor, diode, plate resistor, and power supply element in the amp. All that’s left are a few low-wattage carbon resistors. I had to re-work a botched variable-bias mod. The tube sockets got re-riveted with 6550-size spring retainers. Took about 19 hours.
Did I mention it sounds fantastic now? Very quiet and stable.
Following is a stage-by-stage analysis of the circuit of my particular 1977 master-volume V-4. It’s also applicable to the earlier non-master V-4s and to the V4B, but not to the distortion version.
Notes on tubes are at the bottom.
Here’s something for the pedal nerds. This is a pair of back-to-back clipping diodes connected in parallel, like you find in the Rat distortion pedal. One diode clips the positive-going side of the wave, the other clips the negative-going side. They’re fed by a .047uF coupling cap and 1K series resistor.
The scope trace shows a large sine wave (8Vpp), which is the input signal before the coupling cap. The smaller signal is the clipped output. The negative side has the diode connected directly to ground, so the clipping is hard and fairly rectangular. The positive side has a 390-ohm resistor in series with the diode to ground, so the clipping is much softer.
What happens is: The negative side shunts to ground directly when the amplitude exceeds the diode’s turn-on voltage. The positive side forms a voltage divider with the 1K series resistor and the 390R shunt resistor, when the amplitude exceeds the turn-on voltage. Compression! Easy way to broaden the knee of a diode clipper.
Comparison of the typical Blackface Fender tonestack (blue trace) with the Hiwatt DR tonestack (green trace). The limits are displayed — all combinations of minimum and maximum rotations.
